1. Field of the Invention
The invention relates to techniques for observing properties of bodies of water at a fixed location, particularly observations in regions of great depth over a period of time.
2. Description of the Related Art
Measurements of physical and chemical attributes of water bodies are commonly made by lowering instruments from a surface boat to the bottom of an ocean, lake or river. Water depths can range from a few meters to nearly ten thousand meters. The location from which the data is taken is commonly referred to as a hydrographic station. The water properties can change substantially over time. Therefore, the frequency of measurement needs to be commensurate with the rate of change of the water properties.
The frequency that these measurements can be made is governed by the logistics of re-occupying the observation locations or stations. It is now recognized that a number of important ocean circulation and limnological phenomena occur episodically, and that there are short and long-term changes in water properties. More frequent sampling, or sampling initiated by a measured change in the environment, is important and is needed to elucidate ocean or lake behavior.
While such frequent measurements have been extremely difficult to obtain, the limited number of studies that have re-occupied the same site many times over time have proven to be extremely valuable. For example, an investigation by J. R. Lazier in 1980 of the Labrador Sea documented the effect of low-salinity surface water on deep water convection. Undoubtedly, many other important discoveries concerning ocean water circulation await the ability to obtain long term profiles of water properties.
Long-term measurements at fixed locations are generally made using moored instrument systems. At a limited number of preselected depths, an instrument package is affixed to the mooring cable which is anchored to the sea floor. While this is preferable to single observations, the profile over time is severely constrained by a limited number of sampling depths. Since each preselected depth requires its own complete instrument package, the cost to obtain the data increases proportionately to the number of sites selected. Maintaining calibration of many instrument packages is time consuming and expensive.
It has been recognized that a single sensor platform that can move up and down the mooring cable provides distinct advantages by eliminating the need for multiple sensors and multiple tethering systems. It also provides the advantages of a single calibration applicable to all of the measurements. This is particularly important in long term measurements where sensor drift over time may be large compared to the ocean variability.
One such device is made by Brooke Ocean Technology, Ltd. of Dartmouth, Nova Scotia. This apparatus utilizes surface wave motion as an energy source to power a three-mode microprocessor controlled ratchet system. The apparatus can be locked in place, set to free fall, or engaged to climb down the mooring cable. The apparatus requires wave heights of at least 15 cm at a 2 second period to operate. The descent rate is much slower than the climb rate, and the cycle time differs considerably from one cycle to another. This device has a limited depth range over which it can operate, and requires attachment to a surface buoy rather than a subsurface float. This requirement results in reliability problems due to the dynamic forces created by waves.
Detailed sampling of the ocean's vertical property variations is also commonly accomplished by using expendable bathythermographs such as manufactured by the Sippican Corporation of Marion, Mass. and as disclosed in U.S. Pat. No. 3,552,205, issued to Francis and U.S. Pat. No. 5,555,518, issued to Whalen. This sonde has a streamline-shape so that it will free fall at a constant and predictable rate. The instrument package within the device is connected by thin insulated copper wire which unspools as the device drops, thus providing a profile of water conditions as the sensors within the device send back data. These devices are capable of depths to one kilometer, that is, only the upper portion of the ocean which has an average depth of 31/2 kilometers. This type of device is poorly suited for time series measurements since it requires that a ship stay at the station in order to collect the data. Also, since the device is expendable, the cost of sampling becomes prohibitive for studies that require frequent sampling.
U.S. Pat. No. 4,924,698, issued to Echert et al. on May 15, 1990, discloses an ocean profiler that is designed to be used under pack ice via a tether connected to a stationary surface buoy. The apparatus moves up and down its tether via a variable pitch hydrofoil "wing". The "wing" flies up or down in response to the ocean currents. As with the previously described device, a constant rate of movement up or down or a constant cycle period is not possible since the ocean currents are as changeable as wave height. Use of this device is limited to areas with significant current shear. Depths to 300 meters were typical in the Echert disclosure.
Other approaches to ocean profiling have been attempted. One approach has been the use of a device that adjusts its buoyancy to provide movement up or down along the mooring line via compressed gas. Representative of this genre is the CYCLESONDE invented by J. W. Van Leer et al, as described in Deep Sea Research., vol. 21, pp. 385-400, 1974. Another example is the Webb Research Corp.'s "SLOCUM" is an example of a freely drifting vehicle which can ascend and descend by changing buoyancy.
A buoyancy variant using a pump to move a working fluid into and out of the device is the profiler that was described by Eriksen et al. in the Journal of Geophysical Research, Vol. 87, No. C10, pp. 7879-7902, Sept. 20, 1982. This device, referred to as a profiling current meter has its buoyancy adjusted by computer control. The design is said to decouple the instrument from the vertical motions of the mooring induced by surface waves. Its depth range is limited to a few hundred meters.
There is not found in the prior art an apparatus that moves at a constant speed along its mooring cable, either up or down, that has a cycle time, i.e., from top to bottom and back, which corresponds to the spectrum of changes in the water body being measured, that can profile over the full depth of the ocean, that can profile 1,000,000 meters on a single internal battery, that can control its motion completely, that orients into the current, and has the reliability and energy requirements suitable for making measurements over a long period of time while unattended.